The vast majority of the portable and handheld wireless devices feature nowadays an internal antenna. Internal antennas, particularly those in charge or providing connectivity for cellular services (e.g. 2G, 3G and 4G services such as GSM, CDMA, WCDMA, UMTS, LTE operated within their corresponding frequency bands) require their customization for each model of wireless device as the shape of the device and its radioelectric specifications usually vary from model to model. On the other hand, it is a conventional wisdom that antennas need to keep a certain size with respect to the wavelength in order to radiate efficiently. Therefore, current internal antennas including patches (e.g. PIFAs), IFAs, monopoles and related antenna modules feature a size or length proportional to an operating wavelength of the device, quite typically on the order of a quarter of such operating wavelength. In practice this means that existing internal antennas, internal antenna modules and alike are about the size of the shortest edge of mobile phone (about 35-40 mm for a typical phone, between 40-55 mm in the case of a smartphone). Such a size is particularly inconvenient as the space inside a mobile device is severely limited. Particularly during the design process, the integration of the antennas inside the device becomes a cumbersome task due to the many handheld components such as displays, batteries, speakers, vibrators, shieldings, and the like that compete for real-state with the antenna. The electromagnetic fields radiated by an antenna are quite sensitive to such neighboring components, which makes the design process even more difficult and slow, as addressing all these issues usually involves multiple design iterations. Finally, the fact that the antenna is sizable and not standard in shape makes its integration in an automatized manufacturing process particularly challenging, which means that most of the time the assembly of the antenna inside the device is done manually.
Developing a small, standard antenna that would fit inside every single handheld device would overcome many of the problems related to the handset design and manufacturing process. However, it is well known that reducing the antenna size to make it fit in every handheld severely limits its performance, namely bandwidth and efficiency. H. Wheeler and L. Chu, in the 1940's, first described the fundamental limits on small antennas. They defined a small antenna as an antenna fitting inside a radiansphere, that is, an imaginary sphere of a diameter equal to the longest operating wavelength of the antenna divided by pi (half an sphere in case of unbalanced antennas such as monopoles). They concluded that below such a limit, the maximum attainable bandwidth scales down with the volume of the antenna relative to the wavelength volume (being the wavelength volume a cube volume having an edge length equal to one operating wavelength). In the limit, when the antenna becomes much smaller than the wavelength, it radiates so inefficiently that it can hardly be considered an antenna anymore.
In order to develop a standard radiation system featuring an easy integration into wireless handheld devices, patent applications WO 2010/015365, WO 2010/015364, WO 2011/095330, WO 2012/017013, U.S. 61/661,885, U.S. 61/671,906, disclose for instance a new antenna related technology based on radiation boosters. Such radiation boosters are electrically very small elements (e.g. they feature small volumes fitting inside a cube with an edge about only 1/30 wavelengths and below, typically below 1/50 of the longest operating wavelength), which are in charge of properly exciting the electric currents of a ground plane mode for radiation. Said ground plane is a conductive surface built in the wireless handheld devices, typically including one conductive layer on a printed circuit board which hosts the RF circuitry of the wireless handheld device.
The radiating system in those patent applications further comprises a radiofrequency system (including inductors, capacitors, resistors, and transmission lines) in order to be operative in the desired frequency band or frequency bands such as for example and not limited to LTE700, GSM/CDMA850, GSM900, GSM1800, GSM/CDMA1900, UMTS, LTE2100, LTE2300, LTE2500.
A prior art solution for a radiation booster disclosed, for instance, a solid metal cube as the booster element. Such a cube was designed to feature a very small size compared to the wavelength while minimizing the ohmic resistance losses and reactance of the element. Owing to its small size, a radiation booster supports a significant current density, so a solid, homogeneous, conductive cube option was proposed to minimize the potential losses and reactance and therefore maximize the radiation efficiency of the whole set. Therefore, that embodiment provided a better performance than other boosters that concentrated all the electric current through a single narrow, wire like element. In another test, the miniature solid metal cube was also found to feature a better performance (e.g., bandwidth and efficiency) than a small, conductive thumbtack like booster placed over the ground plane of the wireless device. So in summary, the solid metal cube became over time a preferred solution for an efficient ground plane booster within a wireless device.
Despite said solid conductive cube provided a top performance compared to other booster elements, it still presented multiple problems for real use applications in mass-produced wireless devices, such as for instance: the element was quite heavy owing to the density of its homogeneous metal structure; both the conductive material and manufacturing procedure involving for instance steel mills were far from optimum for producing large quantities of boosters, and from the assembly and integration into the wireless device perspectives, the high thermal conductivity of the booster made it difficult to solder it onto the typical PCB of a wireless device. In addition, due to their physical characteristics, those cubes would not fit well within an automated pick-and-place or SMD processes which are quite typical for PCB electronics manufacturing.